This invention relates to debarking equipment for removing bark from raw logs, and more particularly, to movable debarking blocks that permit variation in the aggressiveness of debarking of logs by variation in the position of the blocks.
In the forest industry, raw logs are converted into many different types of products such as lumber and panel products for construction, and pulp and paper products for newsprint, fine papers and tissues. In manufacturing any of these products, bark must first be removed from the raw logs before further processing can occur.
Drum or rotor debarkers have been developed to remove bark rapidly and efficiently from logs on a large scale. In such debarkers, bark is removed from the logs primarily by mechanical abrasion as the logs rub against each other within an enclosure and against abrading projections known as flights or debarking blocks fixedly mounted within the enclosure. This rubbing action is initiated by turning a drum which defines the enclosure or by rotating one or more rotors which extend into the enclosure. The rotating action of the drum or rotors tends to cause logs within the enclosure to move in a generally circular, tumbling motion to establish the mechanical abrasion described above.
The amount of abrasion required to effectively remove bark from a log depends on factors such as the log species, age, diameter, moisture content and temperature. For effective debarking, abrasion levels must be high enough to remove all the bark while causing only minimal damage to the underlying wood surface.
In conventional drum or rotor debarkers, the extent of debarking action can be controlled by changing the rotational speed of the drum or rotors or by adjusting the amount of time the logs spend in the machine. Our co-pending U.S. patent applications Ser. No. 10/194,696 filed Jul. 12, 2002 (Internal ref: 42768-102) and Ser. No. 10/194,599 filed Jul. 12, 2002 (Internal ref: 42768-103) disclose alternative arrangements for varying the aggressiveness of debarking involving reversing the rotational direction of a drum debarker and providing movable debarking plates in a drum debarker.
Under certain conditions, the amount of debarking action needs to be increased. In conventional practice, fixedly mounted debarking blocks with roughened or serrated surfaces are used to promote abrasion and rotation of the logs. It is well known in the art that the shape and positioning of these debarking blocks can be varied to obtain more aggressive or gentler debarking. Problems, however, arise when wood properties vary from hour to hour or batch to batch of logs. This often occurs in spring or fall, when some logs may be frozen while others are not. Debarking blocks that work well for unfrozen logs will not remove bark from frozen logs. If more aggressive blocks are used for frozen logs, unfrozen logs are damaged as excess wood is removed with the bark and valuable wood fibre is lost.
Another problem with fixedly mounted debarking blocks is that the amount of debarking action can only be varied by changing the blocks. This can take several hours resulting in considerable loss of production as the process involves emptying the debarker of logs and removing and replacing each block manually.
To address the foregoing problems with prior art debarking equipment, the inventors have developed a retractable debarking block that allows for the debarking action to be adjusted during machine operation. The debarking block of the present invention includes an actuator that is used to extend or retract the block to expose or conceal an abrading surface.
Accordingly, the present invention provides a retractable debarking block for a debarker having an enclosure to receive logs for debarking comprising:
a block member having a debarking surface, the member being positionable within the enclosure to be movable between a retracted position in which the debarking surface does not contact logs in the enclosure and an exposed position in which the debarking surface is exposed to engage logs; and
an actuator to move the debarking surface between the retracted and exposed positions, the actuator being adapted to permit automatic movement of the debarking surface toward the retracted position if the force exerted by the logs exceeds a pre-set limit.
In a further aspect, the present invention provides a retractable debarking block for a debarker having an enclosure to receive logs for debarking comprising:
a block member having a debarking surface pivotally mounted within a cavity openable into the enclosure to be movable between a retracted position in which the debarking surface is positioned within the cavity and does not contact logs in the enclosure and an exposed position in which the debarking surface is extended out of the cavity to engage logs; and
an actuator to move the debarking surface between the retracted and exposed positions, the actuator being adapted to permit automatic movement of the debarking surface toward the retracted position if a force exerted by the logs exceeds a pre-set limit.
The present invention also provides a debarking system for a debarker having an enclosure to receive logs for debarking comprising:
debarking means positionable within the enclosure and movable between a retracted position in which the debarking means does not contact logs in the enclosure and an exposed position in which the debarking means engages the logs; and
actuating means to move the debarking means between the retracted and exposed positions and adapted to permit automatic movement of the debarking means toward the retracted position if a force exerted by the logs exceeds a pre-set limit.
The apparatus of the present invention is useful in both rotor or drum style debarkers.
Preferably, the actuator comprises a member such as an airbag or telescoping cylinder that is connected to a pressure reservoir. By activating the member over a range of pressures, the extent of exposure of the debarking block can be varied to adjust the aggressiveness of debarking. As well, the range of pressures allows adjustment of the pre-set force limit beyond which the debarking block automatically moves toward the retracted position to prevent damage to the underlying wood of the logs.
In an alternative arrangement, the actuator comprises a spring member which moves the debarking block by default to the exposed position. Depending on the spring constant of the spring member and the extent to which the spring is compressed, the pre-set force limit can be set according to the type of logs being debarked.